Plasma concentrations of tramadol after transdermal application of a single metered dose of a compounded tramadol gel to cats.

Objective: To determine plasma tramadol concentrations in cats following a single dose of oral and transdermal formulations and the pharmacokinetics for and the concentration of tramadol in the transdermal formulation.

Animals: 8 healthy client-owned domestic shorthair cats.

Procedures: 1 cat was orally administered 1 dose of tramadol (2 mg/kg), and 7 cats received 1 dose of a proprietary compounded tramadol gel product (median actual dose, 2.8 mg/kg) applied to their inner pinnae. Plasma tramadol concentrations were measured with high-performance liquid chromatography-mass spectrometry at fixed times over 24 hours.

Results: Plasma tramadol concentrations were undetectable or much lower (range, < 1 to 4.3 ng/mL) following application of the transdermal formulation, compared with those following oral administration (maximum plasma tramadol concentration, 261.3 ng/mL [at 4 hours]). Tramadol pharmacokinetics for the transdermal formulation could not be determined. Tramadol concentrations of the transdermal gel product exceeded the estimated label dose in all analyzed gel samples, with concentrations greater than the 90% to 110% United States Pharmacopeia standard for compounded drugs.

Conclusions And Clinical Relevance: Application of 1 dose of the proprietary transdermal formulation did not yield clinically relevant plasma tramadol concentrations in cats. Although this proprietary formulation is currently available to prescribing veterinarians, it should be used with caution.